3rd generation (3G) systems, such as the Universal Mobile Telecommunication System (UMTS) have been developed and deployed to further enhance the communication services provided to mobile users compared to those communication services provided by the 2nd generation (2G) communication system known as the Global System for Mobile communication (GSM). In such 3G systems, distinct domains or networks have been identified for Radio Access Networks (RANs) which communicate with the mobile devices. These domains include the circuit switched (CS) domain and the packet switched (PS) domain. In the CS domain signals are physically routed to the appropriate destination through a unique connection whereas in the PS domain message packets are routed to the appropriate destination based on addresses in the packet. So for example, a UMTS CS domain is the UMTS RAN (known as UTRAN) and core network components that provide CS services and a UMTS PS domain is the UTRAN and core network components that provide PS services.
Other IP-based communication systems, such as wireless LAN (WLAN), Worldwide interoperability for Microwave Access (Wi-MAX), Wi-Fi, Long Term Evolution (LTE) systems, provide communication via a PS domain. An IP Multimedia Subsystem (IMS) is a subsystem of a communication system that provides IP multimedia services with PS communication (that is, via the PS domain).
As is well known, cellular communication systems, such as UMTS, provide communication to mobile devices via a plurality of cells, with each cell served by one or more base stations. The base stations are interconnected by a fixed network which can communicate data between the base stations. A mobile device communicates via a radio communication link with a base station of the cell within which the mobile station is situated. In UMTS, the base stations which are part of the UTRAN are known as Node Bs and a mobile device is known as User Equipment (UE).
In order to extend coverage and capacity indoors, such as in residential or small business environments and especially where access would otherwise be limited or unavailable, systems with smaller sized cells served by small base stations, known as femtocells, have been developed. The femtocell incorporates the functionality of a typical base station and some network functionality to allow a simpler, self contained implementation. Current femtocell designs can typically support two to four active mobile devices in a residential setting and thus, are typically used for a closed subscriber group (CSG) or private cell where only subscribers in the group may communicate via the femtocell (also known as private base station). Different architectures for femtocells have been proposed. For example, a UMTS femtocell architecture contains a Home Node B (HNB), a 3G HNB Gateway (3G HNB GW), which interfaces with the UMTS PS and CS domains. The third Generation Partnership Project (3GPP) refers to a 3G femtocell as a Home Node B (HNB) and is working currently to complete a new HNB standard for Rel-8 of specifications: see for example, the 3GPP document TS 25.467 (UTRAN Architecture for 3G HNB). In addition, 3GPP is working to specify an enhanced HNB architecture in the context of Rel-9: see for example, the 3GPP documents TR 23.830 and TR 23.832.
3GPP has defined architecture to support access to the PS domain and to the CS domain of one or more core networks through HNBs. FIG. 1 is a simplified diagram showing one HNB 10 serving a private cell 12, and a Node B (NB) 14 serving a larger cell 16 (referred to as a macro cell). UE 13 communicates with the HNB 10 over a radio communication link 15 and the HNB 10 communicates with a 3G HNB gateway 18 via a Iuh interface 20. NB 14 is coupled to Radio Network Controller (RNC) 22 as is well known in the art. Services are provided to the UE 13 via the CS domain 23 using the lu-cs interface and the Mobile Switching Centre (MSC) 24. Services are provided to the UE 13 via the PS domain 25 using the lu-ps interface and the Serving GPRS Support Node (SGSN) 26 and the Gateway GPRS Support Nodes (GGSN) or Packet Data Network Gateway (PGW) 28. For UEs having IMS capability, access to IMS services may be provided using IMS elements of the IMS 27, the lu-ps interface and the SGSN 26 and the GGSN/PGW 28.
3GPP has recently started a new study item (TR 23.832) on “IMS aspects of architecture for Home Node B” which aims at specifying architectural solutions that enable an operator to offload CS traffic to the IMS. Operators expect the number of HNBs to increase significantly in the future and the minutes of usage per user to also increase significantly due to better coverage in-house and due to fixed-line replacements. To handle the increased voice traffic, the operator can either continue to invest in legacy CS networks or invest in carrying the traffic from the HNB to the IMS and thus, offload CS traffic from the existing CS networks.